Understanding an overlooked hotspot for carbon emissions

In Southeast Asia, the construction of canals to drain water from soggy peatlands unlocks far more planet-warming carbon dioxide than previously estimated, according to a study published March 8 in Nature Geoscience.

“We were fascinated to learn that the drainage canals themselves are a hotspot for peat carbon to be transformed into carbon dioxide,” said study co-author Alison Hoyt, assistant professor of Earth system science at the Stanford Doerr School of Sustainability.

Peatlands cover just 3% of Earth’s land surface, but they store twice as much carbon as all the world’s forests combined. They form in places where year-round flooding prevents dead plants from fully breaking down by limiting their exposure to oxygen. These water-logged conditions allow dead plants – and the carbon that they absorbed from the atmosphere while growing – to accumulate in the peat soil over hundreds or even thousands of years.

But human activities have damaged or destroyed many of the world’s peatlands. In Southeast Asia, people have deforested and drained nearly 60 million acres of peatlands over the past three decades, largely for palm oil and timber harvest, leaving only 6% untouched. Draining and damaging peatlands exposes their accumulated plants to oxygen, causing them to decompose and release carbon dioxide (CO2). Globally, degraded peatlands account each year for around 5% of human-caused greenhouse gas emissions.

“These are some of the largest stores of carbon in the world outside of the ocean and they’ve been locked up for thousands of years,” said study co-author Lihini Aluwihare, a chemical oceanographer at University of California San Diego’s Scripps Institution of Oceanography. “Reintroducing any of that carbon into the atmosphere is of major concern when it comes to climate change. That’s why it’s so important that we figure out what controls the release of carbon from disturbed peatlands.”

Previous tallies of the carbon emissions from degraded peatlands have largely focused on the emissions from dried out peat soils and rarely accounted for the carbon released into waterways.

“We know that damaged peatlands are releasing large amounts of carbon dioxide,” said lead study author Jennifer Bowen, a postdoctoral scholar at Scripps Institution of Oceanography and in Hoyt’s lab at Stanford. “But what happens to the carbon flowing through drainage canals before it reaches rivers or the ocean is less understood. If we don’t know what’s happening there, we could be missing carbon that is entering Earth’s atmosphere and should be included in carbon budgets.”

Related: Global carbon emissions from fossil fuels reached record high in 2023

Indonesia’s peatland canals

To understand what happens to carbon released from peatlands into waterways, the researchers collected water samples from peatland canals in West Kalimantan, Indonesia. In a laboratory, Bowen measured how quickly microbes in the canal water could break down organic matter in the samples and how much carbon dioxide they produced in the process. In additional experiments, the team measured how quickly sunlight caused organic matter in the samples to break down and give off carbon dioxide.

Once the scientists figured out the rate at which canal water exposed to microbes and sunlight produced carbon dioxide in the lab, they assessed the factors likely to accelerate or slow carbon emissions from peatland drainage canals in the real world. Their experiments suggest that in Southeast Asia, sunnier days, higher oxygen concentrations in the canal water, and high levels of mixing within canal waters can lead to higher emission rates.

Based on the results of the experiments, the team estimated that each square meter of canal area in the region releases an average of roughly 70 milligrams of carbon dioxide per day. More work is needed to identify where rates may be higher or lower on the landscape, said Bowen, but the results suggest breakdown by sunlight and microbes may send around 35% of the peat carbon that dissolves into the drainage canals into the atmosphere as carbon dioxide.

“This is the first time anyone has quantified these processes in a tropical peatland, and 35% of the carbon dissolving into these canals is a lot of emissions,” said Aluwihare. “To me, this says that these canal systems are likely a significant source of carbon dioxide emissions on top of the emissions from dried out peat soils, and we are probably underestimating the climate impacts of degrading these systems.”

Aluwihare added that the significant carbon emissions in the peatland canals are likely diminishing the amount of dissolved peat being exported to the ocean. While the fate of peat carbon flowing into the oceans isn’t entirely understood, some may end up being stored again in the marine environment. If that’s the case, Aluwihare said, these results suggest the oceans can’t do as much as scientists thought to prevent peat carbon from re-entering the atmosphere.

Related: Satellite data can reveal fire susceptibility in peatlands

This story was adapted from a press release originally published by the Scripps Institution of Oceanography.

Hoyt is also a center fellow, by courtesy, at the Stanford Woods Institute for the Environment. Additional co-authors on the study are affiliated with the Universitas Tanjungpura in Indonesia.

This research was supported by University of California San Diego’s Scripps Institution of Oceanography and the Precourt Institute for Energy, which is part of the Stanford Doerr School of Sustainability.

Learn more: Read a "News & Views" article about this research in Nature Geoscience